The invention relates to injecting an aqueous solution into a subterranean reservoir to displace fluid within the reservoir. It is useful in substantially any well-treating or oil-recovery process in which reservoir fluid is displaced, particularly in a relatively tight and hot reservoir.
Known oil-displacing processes use aqueous solutions thickened with water-soluble polymers. Such a process was described in the K. D. Ketling Pat. No. 2,341,500 applied for in 1942. Block copolymers containing polymer chains or blocks of varying water-solubility are used in aqueous solutions for various purposes. For example, surfactants comprising block copolymers of polyoxypropylene and polyoxyethylene blocks are described in U.S. Pat. No. 3,203,955. Strong and relatively flexible solid ion exchange resins, of sulfonated polyvinyl arene and alpha-olefin elastomeric blocks, are described in the D. L. E. Winkler U.S. Pat. No. 3,577,357. Block copolymers containing polystyrene and polyisoprene polymer chains and the products of partially hydrogenating such polymers, to reduce most or all of the diene double bonds and provide particularly elastomeric and chemically stable rubber-like materials, are described in the M. M. Wald and M. G. Quam U.S. Pat. No. 3,595,942.
In an oil recovery process in which a polymer-thickened aqueous solution is injected into a subterranean reservoir, the polymeric water thickener is often subjected to shear-degradation. The solution is forced to flow through well conduits and into the relatively fine pores of the reservoir. The reservoir often has a permeability of less than 1 darcy. When fluids are injected at commercially feasible rates, the shearing effects are often significant. Further, in such reservoirs the thickener is usually subjected to hydrolyzing conditions. For example, the thickener may be heated at from about 120.degree. to 180.degree.F under pressures of at least several hundred pounds per square inch for times of from about 4 to 10 years. The water in such reservoirs often contain salts that tend to precipitate a polymeric water thickener or decrease its viscosity-enhancing ability.
Because of the severe physical and chemical requirements, plus a need for a relatively low cost, only a few polymeric water thickeners have been tested in oil recovery processes. Those are polyacrylates, such as partially hydrolyzed polyacrlamides, and polysaccharides such as the xanthan polymers. The polyacrylates have relatively low shear and chemical stabilities and the polysaccharides contain pore-plugging particles that are difficult to remove. In SPE Paper No. 4748, by Hill, Brew, Claridge, Hite and Pope, presented at the April 1974 Symposium on Oil Recovery, it is indicated that the polyacrylamides are subject to mechanical degradation and may be severely degraded when injected through restricted entry completions at some field rates. Mobility control mechanisms are complex and require careful evaluation for each particular application. As shown by Jennings et al. (Journal Petroleum Technology, March 1971, 391-401), use of polyacrylamide polymers in high permeability rocks may require large concentrations of polymer to achieve adequate mobility control. This situation is particularly aggravated when high permeability rocks and a highly saline aqueous phase are both a part of the total environment of the application. Finally, stability of polyacrylamide solutions for long reservoir times at high temperatures (above 140.degree.F) has not been adequately established and/or documented.
Biopolymers offer some potential advantages (1) the mobility control mechanism appears to be that of viscosity building alone; thus, design and monitoring are simplified; (2) the viscosity, as shown by Jennings (above), is not as sensitive to high salt concentrations; and (3) the solutions are not as sensitive to mechanical degradation as polyacrylamide solutions. But, offsetting these potential advantages of the biopolymers, are questions regarding filterability-injectivity, gel formation in aqueous environments containing multivalent cations and long-term stability or resistance to chemical alteration in the reservoir environment.